Multiple-cam, posterior-stabilized knee prosthesis

ABSTRACT

A distal femoral knee-replacement component provides additional points of cam action by either distinct bars or interconnected structural elements such as cam extensions to prevent early translation of the knee or dislocation of the femoral component over the tibial post which can occur in cruciate-substituting designs.

REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.09/724,100, filed Nov. 28, 2000, now U.S. Pat. No. 6,558,426, the entirecontent of which is incorporated hereby by reference.

FIELD OF THE INVENTION

This invention relates generally to orthopedic surgery and, inparticular, to a posterior stabilized knee prosthesis.

BACKGROUND OF THE INVENTION

In total knee-replacement (TKR) surgery, there are four broad classes ofimplants used for resurfacing of the distal femur. In one configuration,the posterior cruciate ligament is retained. In another design theligament is sacrificed, relying on the articular geometry to providestability. The third type of device is constrained, in the sense that anactual linkage is used between the femoral and tibial components.According to a fourth arrangement, the posterior cruciate is replacedwith a cam on the femoral component and a post on the tibial component.

Many patents have been issued in relation to these designconfigurations, including the cam-and-post design configuration. Some ofthe earlier patents in this area include U.S. Pat. No. 4,213,209 toInsall et al; U.S. Pat. No. 4,298,992 to Burstein et al.

Other patents include U.S. Pat. No. 4,888,021 to Forte et al., whichteaches a cam-and-post mechanism as well as a linking mechanism.Essentially, each component includes a varying surface and a cam member,so that both the tibial and the femoral component have separate anddistinct cams that cooperate with a single tibial post.

U.S. Pat. No. 5,824,100 to Kester et al. discloses a cam/post type ofarrangement with a unique type of cam and box enclosure. A portion ofthe box enclosure is intended to prevent hyperextension and posteriortranslation. As noted in particular in FIGS. 3 and 4 of the '100 patent,a large space exists between the cam 110 and the post 100 which permitsa translation to occur prior to engagement of the cam left of post.

U.S. Pat. No. 5,997,577 to Herrington et al. provides a cam on the femurwith a geometry meant to contact the post through a large range ofmotion. This design attempts to provide the function of multiple cams byproviding an area that acts as a separate bearing surface. As such, thecam effectively moves through a range of motion while contacting thepost. Depending on the articular geometry which differs than thegeometry of the cam post mechanism, this could lead to a variety ofproblems as well as significantly constrained motion, either between thecam and the post or between the two articulating surfaces.

U.S. Pat. No. 5,658,342 to Draganich et al. describes a cam member withincluding a bearing surface at complimenting an articulating surface. Asin other previous designs, this represents a complex cam geometry meantto capture the post in certain degrees of the range of motion.

U.S. Pat. No. 5,147,405 to Van Zyle et al. Discloses a femoral componentwith two distinct cam structures, one located at point 44, the otherlocated at 46 in the drawings. The cam member 44 is meant to contact theanterior surface of the post 24 to prevent hyperextension, while camsurface 46 is a posterior located cam meant to have contact throughoutthe range of flexion. As noted in FIG. 6A of the '405 patent, there is aspace between the cam and the post when the knee is in extension,necessitating anterior translation of the femur on the tibia prior tocontacting the posterior cam.

Many other patents directed to knee-replacement surgery includecam-and-post mechanisms. But in all cases, either the full range ofjoint motion is precluded, or translation is allowed to occur whichcould lead to premature wear. FIG. 1 is a drawing which illustrates atypical prior-art cam-and-post mechanism. Item 102 is a tibial insert ortibial component having a post 103 protruding into a box-like recess ofthe femoral component 100. FIG. 1A shows the system in extension,whereas FIG. 1B shows the system in flexion. In FIG. 1A, a femoralcomponent 100 includes a cam 101 which has not yet engaged with a post103.

In FIG. 1B, following a considerable amount of flexion, the cam 101finally engages with the post 103. Until engagement occurs, however, thecomponent 100 may be permitted to slide relative to the tibial insert.The need remains, therefore, for an improved distal femoral prosthesishaving multiple distinct cams contacting a post on its posterior surfaceto provide a more normal range of motion for cruciate substituting kneereplacement.

SUMMARY OF THE INVENTION

The present invention resides in a distal femoral knee-replacementcomponent configured for use in a cruciate-substituting situationinvolving a tibial component having a bearing surface and a superiorpost with a posterior aspect. As with existing configurations, thecomponent is comprised of a body having a pair of medial and lateralcondylar protrusions and an intercondylar region therebetweendimensioned to receive the tibial post. In contrast to prior-artdevices, however, the inventive component provides additional points ofcam action to facilitate a more normal range of knee motion.

In the preferred embodiment, the invention facilitates a more normalrollback while inhibiting initial translation which could lead toincreased wear and sub-optimal patella femoral mechanics. To accomplishthis goal, the inventive component includes a distinct point of camaction to prevent early translation at the initiation of flexion, and adistinct point of cam action to prevent a dislocation of the femoralcomponent over the tibial post which is known to occur incruciate-substituting designs. According to the invention, these pointsof cam action may be used separately or in combination.

In the preferred embodiment, the component includes three distinctpoints of cam action. The first is preferably located substantiallywhere existing cams are found, namely, at a point spaced apart a slightdistance posteriorly relative to the post in full extension. Accordingto the invention, however, a second point of cam action is locatedimmediately adjacent the posterior aspect of the superior post tominimize and, ideally, prevent anterior translation at the initiation offlexion. The third point of cam action is preferably located moreposteriorly to allow enhanced flexion without a dislocation of the knee.

In terms of structure, the points of cam action may be implemented usingany member or combination of elements operative to provide distinctstages of cooperation with the posterior aspect of the superior post.For example, transverse bars may be used which bridge, or partiallybridge, the intercondylar space. The members or elements need not bestraight across, but may instead be curved, with the post being curvedto allows for a rotation, if so desired. The cam structures according tothe invention may also be connected to one another forming points ofcontact as opposed to complete transverse elements such as distinctbars.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a drawing which illustrates a prior-art cam-and-postmechanism in extension;

FIG. 1B is a drawing which illustrates the prior-art cam-and-postmechanism of FIG. 1B in flexion;

FIG. 2A illustrates a preferred embodiment of the invention inextension;

FIG. 2B shows the system of FIG. 2A at 90 degrees flexion;

FIG. 2C illustrates the system of FIG. 2A in flexion at 120 degrees ormore;

FIG. 2D illustrates the alternative use of interconnected cams withphysically separate contact points;

FIG. 2E illustrates the distal femoral component of FIG. 2D and the postconfiguration of FIG. 6A at full extension;

FIG. 2F shows the distal femoral component of FIG. 2D and the postconfiguration of FIG. 6A at 90 degrees of flexion;

FIG. 2G illustrates the distal femoral component of FIG. 2D and the postconfiguration of FIG. 6A in flexion beyond 90 degrees; and

FIG. 2H shows a degree of hyperflexion wherein the extended cam in FIG.2D first makes contact with the posterior surface of the post.

FIG. 3 shows an anterior view of a prior-art cruciate-substitutingknee-replacement component;

FIG. 4 shows a knee prosthesis according to the invention havingmultiple cams as seen in a distal-to-proximal view;

FIG. 5 is a drawing which shows how cam-acting members according to theinvention need not be straight across, but may be curved in conjunctionwith a curved post to facilitate rotation; and

FIGS. 6A through 6E illustrate various alternative post configurations,all of which are applicable to the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 2A through 2C illustrate one embodiment of the invention. FIG. 2Ashows the configuration in extension, FIG. 2B shows the system at 90degrees flexion, and FIG. 2C illustrates flexion of 120 degrees or more.In addition to a conventionally placed cam at 101, two additional pointsof cam action are preferably provided. In particular, a feature at 201acts to prevent translation from extension into the initiation of theflexion. Feature 201 preferably disengages as conventional cam 101 isengaged. As the knee follows through a range of motion to 90° offlexion, and beyond, cam 101 disengages and feature 202 engages, ifnecessary, to prevent dislocation of the component.

In FIG. 2B, the cam which is usually present at 101 is engaging theconvex tibial post 103, cam 201 has disengaged, and cam 202 has not yetengaged but is available for engagement on further flexion. In FIG. 2C,cam 202 is now engaged the post in the presence of additional flexion.Cam 101 can now disengage, cam 201 had disengaged earlier. FIG. 2Dillustrates the alternative use of interconnected cams with physicallyseparate contact points.

In FIGS. 2A through 4, the features depicted to provide the variousstages of cam/pivoting function are depicted as bars which cross theintercondylar recess or box portion of a cruciate substituting designknee. However, although the terms “cam” or “bar” are used to referencethe stages of cam action, it should be understood that the responsiblestructures may be implemented using any member or combination ofelements operative to provide distinct stages of cooperation with theposterior aspect of the superior post. Thus, the members or elementsneed not be complete or straight across, but may instead be curved, withthe post being curved to allow for a rotation, if so desired. The camstructures according to the invention may also be connected to oneanother forming points of contact as opposed to complete transverseelements. The structure may be provided as part of an open- orclosed-type of a box structure, both being familiar to those of skill inthe art.

Whereas FIGS. 1 and 2 represent lateral or side views of a knee throughvarious ranges of motion, FIG. 3 shows an anterior view of a prior-artcruciate substituting knee component at 300 having an open-type box 302including a single transverse member 301 for illustrative purposes. FIG.4 shows a knee prosthesis 400 according to the invention, viewed againfrom the distal-to-proximal perspective, having three distinct points ofcam action. In particular, cam 401 is conventionally located, ananterior cam is disposed at 402 in support of a greater range ofenhanced flexion, and a more posterior cam at 403 is used primarily toprevent dislocation of the cams over the post, as discussed above.

FIG. 5 is a drawing which shows various cams from a top view lookingdown. Note that bars of need not be straight across, but may be curvedwith the post being curved so that it allows for a rotation to occur ifdesired. The cam structures according to the invention may be individualdistinct bars or may be connected to one another forming points ofcontact as opposed to distinct structures themselves. It should also benoted that the cam structures may be located at different locations fromthe posterior to the anterior aspect of the knee design, as well as fromthe distal or proximal, depending upon implant size, patient physiology,desired range of motion, and other requirements. It should further benoted that as opposed to using three separate cams, one could use twocams intended to contact the posterior aspect of the post or for thatmatter, use more than three if desired.

It will also be apparent to one of skill in the art that the posterioraspect of the post may be modified to affect the timing and/or operationof the cam engagement. FIGS. 6A through 6E illustrate variousalternative post configurations, all of which are applicable to theinvention. As opposed to a substantially straight configuration, asdepicted in FIG. 6A, the post may be posteriorly oriented along theposterior aspect, as shown in FIG. 6B. Alternatively, the post may beanteriorly oriented along the posterior aspect, as shown in FIG. 6C. Asfurther alternatives, the post may be stepped, as shown in FIG. 6D, orcurved, as shown in FIG. 6E. Also, as opposed to the sharp cornersshown, they may be rounded off, and the bars or recesses may be adjustedfrom the positions shown in FIGS. 2 through 5 to achieve a desiredoperation.

1. In a total knee-replacement system including a tibial componenthaving medial and lateral bearing surfaces and a superior post with aposterior surface, and a distal femoral component having medial andlateral condylar protrusions that articulate with the bearing surfacesand an intercondylar region with a central cam that engages with theposterior surface of the superior post, the improvement comprising: adistal femoral component having a central cam with a first, convex camaction surface in the intercondylar region that engages the posterioraspect of a tibial post before 90 degrees of flexion; a cam extensionthat terminates in a second cam action surface that makes initialcontact with the posterior aspect of the post after 90 degrees offlexion to facilitate enhanced knee flexion without dislocation over thetibial post; and an intermediate surface portion between the first andsecond cam action surfaces that does not make contact with the tibialpost.
 2. The distal femoral component of claim 1, wherein the posteriorsurface of the post is substantially flat or curved.
 3. The distalfemoral component of claim 1, wherein the central cam and the camextension are interconnected but provide physically separate contactpoints.
 4. The distal femoral component of claim 1, wherein: the medialand lateral condylar protrusions have a radii of curvature; and the camextension has a radius of curvature substantially less than any of theradii.
 5. The improvement of claim 1, wherein the second cam surface ismore proximal than the first cam surface with the knee in extension. 6.A total knee replacement system, comprising: a tibial component havingmedial and lateral bearing surfaces and a tibial post with a posteriorsurface; a distal femoral component having an intercondylar regionconfigured to receive the tibial post and medial and lateral condylarsurfaces that articulate with the bearing surfaces of the tibialcomponent over a range of motion from extension through flexion; and amember on the distal femoral component bridging the intercondylarregion, the member including: a first, convex cam surface that engageswith the posterior surface of the tibial post following the onset offlexion, and a cam extension with a second cam action surface thatinitially engages with the posterior surface of the tibial post beyond90 degrees of flexion, to minimize dislocation over the tibial post; andan intermediate surface portion between the first and second cam actionsurfaces that does not make contact with the tibial post.
 7. The totalknee-replacement system of claim 6, wherein the posterior surface of thepost is substantially flat or curved.
 8. The total knee-replacementsystem of claim 6, wherein the central cam and the cam extension areinterconnected but provide physically separate contact points.
 9. Thetotal knee-replacement system of claim 6, wherein: the medial andlateral condylar protrusions have a radii of curvature; and the camextension has a radius of curvature substantially less than any of theradii.
 10. The total knee replacement system of claim 6, wherein thesecond cam surface is more proximal than the first cam surface with theknee in extension.
 11. A knee implant for use in posterior cruciatesacrificing procedures, comprising: a tibial component having a superiorpost with a posterior surface; a femoral component having medial andlateral condylar protrusions which form separated bearing surfacesconfigured to articulate with the tibial component and an intercondylarfemoral cam mechanism; the cam mechanism including an intercondylarbridging structure having a convex surface which engages with theposterior surface of the superior post; a cam extension with a separatesurface positioned to engage with the posterior surface of the superiorpost to reduce the risk of dislocation, but only at flexion greater than90 degrees; and an intermediate surface portion between the first andsecond cam action surfaces that does not make contact with the tibialpost.
 12. The implant of claim 11, wherein the cam extension at leastpartially bridges the intercondylar region.
 13. The knee implant ofclaim 11, wherein the surface of the cam extension is more proximal thanthe convex surface with the knee in extension.
 14. A total kneereplacement system, comprising: a tibial component having medial andlateral bearing surfaces and a tibial post with a posterior surface; adistal femoral component having an intercondylar region configured toreceive the tibial post and medial and lateral condylar surfaces thatarticulate with the bearing surfaces of the tibial component over arange of motion from extension through flexion; and a member on thedistal femoral component bridging the intercondylar region, the memberincluding: a convex cam surface that engages with the posterior surfaceof the tibial post following the onset of flexion, and a cam extensionwith a cam action surface that initially engages with the posteriorsurface of the tibial post beyond 90 degrees of flexion to minimizedislocation over the tibial post, and wherein the cam extension projectsproximally away from the tibial articulating surface when the knee is inextension.
 15. The system of claim 14, wherein an additional camextension with a cam action point projects distally toward the tibialarticulating surface when the knee is in extension and contacts theposterior aspect of the tibial post early after the initiation offlexion to minimize early translation of the femur relative to thetibia.
 16. The system of claim 14, wherein the cam surface is curved inthe transverse plane to allow axial rotation.
 17. The total kneereplacement system of claim 14, wherein the surface of the cam extensionis more proximal than the convex surface with the knee in extension.